Daughter just received a "sext." What do we do?

My 15 year old daughter was texting one of her male friends. Based on the messages that I read, he told her that he liked her. She told him that she liked him too but wasn't allowed to date. He responded with a sad face and then sent a picture of his penis before she responded.She came to me immediately and showed me the text. I took her phone away and we talked about what happened. She just went up to bed and I told her that I was going to hold her phone for the night so he doesn't keep texting her.Where do we go from here? I think he is just a dumb kid who made a mistake, but he needs to know that this isn't acceptable. Do I tell his mom? I have a 15 year old son as well and would want to know if he did something like that. I just don't know what to do here. via /r/Parenting http://ift.tt/2ifXYNz